Unhairing hides

ABSTRACT

HIDES TO BE CONVERTED TO LEATHER ARE RENDERED SUITABLE FOR MECHANICAL UNHAIRING IN A TWO-STAGE PROCESS. IN THE FIRST STAGE, THE HIDES ARE SOAKED IN AN AQUEOUS SOLUTION CONTAINING SOLUBLE SULFITE OR BISULFITE AND UREA AT A PH FROM 4.5 TO 9.0 AND AT A TEMPERATURE UP TO 42*C. IN THE SECOND STAGE, A SOLUBLE CUPRIC SALT IS ADDED TO THE SOLUTION IN AN AMOUNT FROM 0.01 TO 0.1 THE MOLAR AMOUNT OF SULFITE PRESENT, THE PH IS ADJUSTED TO 9.8-10.9 WITH AMMONIA, AND THE SOLUTION IS AERATED UNTIL THE HAIR IS SUFFICIENTLY LOOSENED TO BE EASILY REMOVABLE BY COVENTIONAL MECHANICAL MEANS.

United States Patent fice 2 3,582,254 Patented June 1., 1971 3,582,254 UNHAIRING HIDES Martin K. 0. Lindemann, Rockville, Md., and Jacob J.

Guth, Arlington, Va., assignors to The Gillette Company, Boston, Mass. No Drawing. Filed Apr. 16, 1969, Ser. No. 816,781 Int. Cl. C146 1/06 US. Cl. 894.16 3 Claims ABSTRACT OF THE DISCLOSURE Hides to be converted to leather are rendered suitable for mechanical unhairing in a two-stage process. In the first stage, the hides are soaked in an aqueous solution containing soluble sulfite or bisulfite and urea at a pH from 4.5to 9.0 and at a temperature up to 42 C. In the second stage, a soluble cupric salt is added to the solution in an amount from 0.01 to 0.1 the molar amount of sulfite present, the pH is adjusted to 9.810.9 with ammonia, and the solution is aerated until the hair is sufiiciently loosened to be easily removable by conventional mechanical means.

This invention relates to the preparation of hides for unhairing and pertains more specifically to a two-stage process for loosening or weakening the hairs of hides such as cattle hides in a short period of time with minimum damage to hair and to hide.

An essential and early step in making leather from hides of cattle or other animals is the removal of the hair. In order to be satisfactory, an unhairing process must remove the hair including the root portion substantially completely so that substantially none of the hair breaks off above the skin surface to leave a stubble; it must leave the hide unstained, unmarked, and undamaged and must not remove a substantial proportion of the collagen; and it must leave the hide in open relatively permeable condition and receptive to the enzymes employed in the subsequent bating operation to remove undesired constituents such as certain proteins other than collagen. Because of these stringent requirements, conventional unhairing processes, which involve soaking the hide in an aqueous suspension of calcium hydroxide to soften and loosen the hair at or near the root after which it is pulled or scraped from the skin, for example by rotating friction wheels, usually require that the hides be immersed for several days, and attempts to speed up the process by increasing the temperature or by including sodium sulfide or amines in the aqueous medium have met with only limited success, at least twenty-four hours immersion and usually more being required.

It has also been proposed by Swan in Australian Journal of Chemistry 14, 69 (1961) to employ an aqueous solution of sodium sulfite, urea, and ammonia to dissolve keratin. However, the procedures described by Swan are ineifective to produce unhaired hide meeting the requirements set forth above, and to do it in a short period of time.

The process of the present invention does produce unhaired hide meeting the requirements set forth above and renders the hide ready for conventional mechanical unhairing within as short a time as 1 to 2 hours. In addition, hair which has been separated from the hide by the process of the present invention has not been destroyed but is still strong enough for use in the manufacture of fibrous articles such as carpet underlays or cushions.

In the process of the present invention the hides, after desalting and partial fleshing, are treated in the first stage of the process with an aqueous solution containing a reducing agent which is a water-soluble sulfite or bisulfite,

and a urea compound, the hide preferably being simply immersed in the solution at the desired temperature and allowed to remain immersed for the required time period. The Water-soluble sulfites and bisulfites which may be employed either alone or in admixture with each other include alkali metal salts such as sodium or potassium sulfite and bisulfite as well as ammonium sulfite and bisulfite and amine sulfites and bisulfites such as triethanolamine sulfite. Sodium sulfite and particularly ammonium sulfite are preferred. The concentration of the reducing agent may range from 0.2 to 1.5 molar at the beginning of the first stage of the process; during the course of the process the supply of reducing agent is depleted and may approach zero at the end of the second stage.

While urea is the preferred material for use as the second ingredient in the aqueous solution used in the first stage, it is also possible to use methyl urea or ethyl urea. The concentration of these urea compounds in the solu tion at the start of the process may range from 0.5 to 4.0 molar. It is not essential during the first stage of the proc ess to adjust or regulate the pH, and it may simply be left at whatever pH is produced by dissolution of the reducing agent and urea compound, which in the case of sodium sulfite and urea is about 8.5. In general, the pH of the solution during the first stage of the process may range from 4.5 to 9. The relative proportions of sulfite and bisulfite will vary, of course, depending upon the pH of the solution. The term sulfite is used hereinafter, in the generic sense to include varying proportions of bisulfite, depending upon the pH of the solution.

The quantity of solution used during the first stage of the process, i.e. the ratio of the Weight of the solution to the weight of the hides should be at least 2.5 :1. Higher ratios may be employed, approximately 4:1 being preferred, and it will be understood that there is no upper limit on the amount of solution which may be used although it is wasteful, of course, to use more than is needed. While some depletion of the ingredients of the solution occurs during the process, this is not very great and normally does not require any precautions unless the same batch of solution is used for a number of successive hides.

The temperature of the solution during the first stage of the process may be as high as 42 C., but higher temperatures should be avoided because of the risk of excessive shrinking the hide. In general, any temperature from 0 to 42 C. is satisfactory.

The first stage of the process is complete when from 15 to 30% of the available disulfide bonds in the hair have been ruptured by reaction with the reducing agent. The precise time required for completion of the first stage depends upon the interrelationship of several variable factors, the most important of which are the pH and temperature of the solution: the lower the pH within the specified range, the shorter the time required; and the higher the temperature, the shorter the time. In addition, the identity of the cation present in the solution also afi'ects the time required, although to a lesser extent: ammonium sulfite solution requires less time than sodium sulfite under the same conditions. To a still lesser extent, the nature of the hide and hair also afiects the time required, black or red hairs on cattle hides requiring slightly longer times than white hairs, although the difference is not great enough to prevent the use of the process to treat spotted hides. When an ammonium sulfite solution is used at the minimum pH of 4.5 and at the maximum temperature of 42 C., the minimum time required for the first stage is about 20 minutes, assuming that the hide has previously been soaked out and heated to 42 C. If sodium sulfite is employed instead of ammonium sulfite, the minimum time is about 30 minutes. 0n the other hand, using a sodium sulfite solution at pH of 8.5 and a temperature of C. the minimum time required for the first stage is about 2 hours. The first stage of the process may be continued for a substantially longer time than the required minimum without deleterious effects, but if the first stage is prolonged und-uly one of the principal advantages over the conventional process, that of speed, is nullified. While it is possible by conventional analytical procedures to determine the proportion of available disulfide bonds in the hair (usually 0.8 to 1.2 milliequivalents per gram of hair) and also the extent to which such bonds have been broken by the reducing agent, it is usually more convenient and practical to carry out the process in a small scale test (i.e. carry out a pilot process) to determine optimum conditions in any given case. After some experience in carrying out the process it is usually unnecessary to continue to carry out analyses or tests unless unusually great departures from previous experience occur.

It can be stated, in the light of the foregoing, that the time for the first stage can vary in general from minutes to 5 hours or more.

Upon completion of the first stage of the process. there is dissolved in the solution a small amount of water-soluble cupric salt, and the pH is adjusted with ammonia (ammonium hydroxide) to a value from 9.8 to 10.9. The ammonium ion has the effect of solubilizing the cupric salt and of accelerating the loosening of the hair. The ammonium ion may be added together with the cupric ion or separately, in the form of ammonium hydroxide, for example. Any ammonium ion added during the first stage of the process is effective in the second stage, so that if ammonium sulfite, for example, is used in the first stage, less ammonia need be added in the second stage, producing a pH at or near the lower end of the range. On the other hand, if no ammonium ion is added during the first stage, it is desirable for best results to add a larger amount of ammonia in the second stage in order to provide a pH at or near the upper end of the range. The solution is also aerated during the second stage, either by sparging or bubbling air through it or by vigorous agitation or by any other suitable means.

While any soluble cupric salt may be employed, cupric sulfate and cupric tetrammine sulfate are preferred: cupric chloride or bromide, cupric nitrate, cupric lactate,

cupric acetate or cupric formate may also be used. The

concentration of cupric salt may be from 0.01 to 0.1 the molar concentration of the sulfite present at the beginning of the process. The greater the concentration of the cupric salt within the specified range, the more rapidly the process proceeds; but if the molar concentration of the cupric salt is greater than 0.1 times the concentration of the soluble sulfite, an oxidation-reduction reaction tends to occur which slows down the process. The exact concentration of cupric salt required for optimum results in any particular case will also depend to some extent upon the manner in which the first stage of the process was carried out and in particular upon the concentration of the urea compound. When the concentration of the urea compound is near the upper end of the specified range, it is desirable to use a concentration of cupric salt near the lower end of its range in order to avoid excessive penetration of the skin and consequent staining by the copper ions. The temperature of the solution, and the solution/hide ratio may be in the same ranges in the second stage as in the first. The time required for the second stage may be as short as about 30 minutes or even less. The condition of the hide is examined at intervals during the second stage in order to determine by inspection when the hair has become suffiriently loosened and weakened so that it may be removed by conventional procedure and apparatus e.g. by

friction wheels.

The following specific examples are intended to illustrate more fully the nature of the invention without serving as a limitation upon its scope.

EXAMPLE 1 A piece of wet desalted Hereford hide weighing 3 kg. was immersed in 12 liters of a water solution containing 1'56 g. of sodium sulfite (0.5 molar) and 1.08 kg. of urea t 1.5 molar). The pH of this solution was 8.6. The hide was immersed with occasional stirring for three hours at C. At this time, there were added to the solution 144 g. of concentrated ammonium hydroxide (28% ammonia) and 150 g. of cupric sulfate pentahydrate and the solution was stirred until the cupric salt was completely dissolved to provide a concentration of 0.05 molar. A further amount of 72 g. of concentrated ammonium hydroxide was added and stirred into the solution to adjust the pH of the solution to 10.6. Air was then bubbled into the solution through a sparging tube and the temperature was maintained at approximately 25 C. for three more hours. At this point, the hair on the hide was sufliciently loosened so that it could readily be removed by passing the hide through conventional rotating friction wheels. The unhaired hide was free from mats and blemishes, exhibited no staining, and after pickling in acid responded very well to a conventional bating operation.

EXAMPLE 2 A piece of wet desalted Hereford hide weighing 1.3 kg. was immersed in 5.2 liters of a water solution containing 328 g. of sodium sulfite (0.5 molar) and 470 g. of urea {1.5 molar). The pH of this solution was 8.6. Immersion of the hide continued for 2.5 hours with occasional stirring of the solution at a temperature of 25 to 30 C. There was dissolved in a mixture of 12.6 g. of concentrated ammonium hydroxide together with a small amount of water 13 g. of cupric sulfate pentahydrate, and this solution was mixed with the sulfite-urea solution to provide a concentration of cupric salt of 0.01 molar. The pH of the solution was adjusted to 10.6 by the introduction of a small additional quantity of concentrated ammonium hydroxide amounting to 6.3 g. Air was introduced through the sparging tube and the solution was stirred gently for a period of 2.5 hours while the temperature was maintained at 25-30C. At the end of this time, the hair on the hide was sufficiently loosened so that it could be removed as in Example 1 to provide an unhaired hide having similar characteristics.

EXAMPLE 3 A piece of wet desalted Hereford hide weighing about 1 kg. was immersed for 2 /2 hours at a temperature of C. in 4 kg. of a water solution containing 252 g. of sodium sulfite 0.5 molar) and 480 g. of urea {2.0 molar), the solution having a pH of 8.5. At this point there was dissolved in the solution g. of cupric sulfate pentahydrate together with g. of concentrated ammonium hydroxide to provide a solution having a pH of 10.6 and having a concentration of cupric salt of 0.05 molar. After 30 minutes aeration at a temperature of 40 C., the hair was sufliciently loosened and weakened for mechanical unhairing in the usual manner. The resultant hide possessed the same desirable characteristics as in the case of the preceding examples.

EXAMPLE 4 A piece of wet desalted Hereford hide weighing approximately 1 kg. was immersed in 4 liters of a water solution containing 252 g. of sodium sulfite (0.5 molar l, 480 g. of urea (2.0 molar), and 20 ml. of concentrated hydrochloric acid. The pH of the solution was 6.0 and it was gently stirred for a period of one hour while maintaining the temperature at 40 C. At that time there were stirred into the solution 50 g. of cupric sulfate pentahydrate and g. of concentrated ammonium hydroxide, providing a solution having a pH of 10.5 and a. concentration of cupric salt of 0.05 molar. After aeration of the solution for 30 minutes at a temperature of 40 C., it was found that the hair was sutficiently loosened so that it could readily be removed in the conventional mechanical unhairing step. The hide so produced possessed all of the desirable characteristics of those produced in the preceding examples.

While it is generally desirable to employ the shortest possible time for the process for use on a commercial or industrial scale, there is a considerable margin for variation, and it is generally safest to operate near the middle of the various ranges of concentration, temperatures, and pH range in order to have the greatest latitude in the time of immersion required to provide optimum results.

Although specific embodiments of the invention have been described herein, it is not intended to limit the invention solely thereto but to include all of the variations and modifications which suggest themselves to one skilled in the art within the spirit and scope of the appended claims.

What is claimed is:

1. The method of preparing hides for unhairing which comprises maintaining the hides in contact with an aqueous solution containing a water-soluble salt reducing agent from the group consisting of the alkali metal, ammonia or amine sulfites or bisulfites and a urea compound from the group consisting of urea, methyl urea and ethyl urea, the initial concentration of reducing agent being from 0.2 to 1.5 molar and of urea compound being from 0.5 to 4.0 molar, maintaining the temperature of the solution at 0 to 42 C. and the pH from 4.5 to 9 for a period from 20 minutes to 5 hours at a ratio of solution to hide of at least 2.5 :1 by weight, then adjusting the pH with ammonia to a value from 9.8 to 10.9 and dissolving in the solution an amount of cupric ionizable salt to provide a molar concentration from 0.01 to 0.1 the molar concentration of reducing agent present, and aerating the solution while maintaining the pH from 9.8 to 10.9 and the temperature from 0 to 42 C. until the hairs are sufficiently loosened for mechanical removal from the hide.

2. The method as claimed in claim 1 in which the reducing agent is sodium sulfite, and the pH before addition of cupric salt is about 85.

3. The method as claimed in claim 1 in which the reducing agent is ammonium sulfite.

References Cited UNITED STATES PATENTS 7/1958 Strandine et al. 894.16X 7/1960 Thorstensen 894.16 

